1. Field of the Invention
The present invention pertains to a method for obtaining low invasion core samples from a subterranean formation through a producing well using coilable tubing to run the drilling and coring tools.
2. Background
Producing oil and gas from subterranean formations through wellbores sometimes requires inspection of formation conditions to analyze production characteristics and prescribe future production techniques. Analysis of formation characteristics or changes is usually dependent on the ability to take suitable core samples of the formation in the vicinity of the wellbore. Conventional coring operations require that the well be shut in while a drilling rig is brought in and operated to perform the coring operation. This process is time consuming and expensive and requires shut-in of the well during all phases of the drilling and core sample acquisition process. Improved methods and apparatus for obtaining cores are described in U.S. patent applications entitled: Coring With Tubing Run Tools From a Producing Well by John C. Braden, et al and Permanent Whipstock and Placement Method by David D. Hearn, et al, both assigned to the assignee of the present invention, and both of even filing date with this application.
Another problem associated with obtaining core samples from subterranean formations pertains to reducing invasion of the core by unwanted fluids such as the cuttings evacuation fluid used during the core acquisition process. Low invasion coring has been successful only when the core diameters exceed approximately 4.0 inches. As the core diameter is decreased, the core surface area to pore volume ratio increases so that the invasion of fluids becomes greater. At core diameters of less than about 2.5 inches, coring fluid invasion often reaches the center of the core. Accordingly, this limitation on core diameter has, heretofore, precluded obtaining core samples through small diameter tubing strings and other wellbore structures of a diameter less than conventional casing diameters thereby, again, requiring the use of a drilling rig to pull the tubing strings and provide a drillstring for obtaining a core sample of sufficient diameter to prevent invasion of the core center. However, the present invention overcomes the disadvantages of conventional coring by providing a unique method for obtaining core samples from and through a production well.